As one of methods for synthesizing liquid fuels from a natural gas, a GTL (Gas To Liquids: a liquid fuel synthesis) technique of reforming a natural gas to synthesize a synthesis gas containing a carbon monoxide gas (CO) and a hydrogen gas (H2) as main components, synthesizing hydrocarbon compounds (FT synthesis hydrocarbons) using this synthesis gas as a feedstock gas by the Fischer-Tropsch synthesis reaction (hereinafter referred to as “FT synthesis reaction”), and further hydrogenating and fractionally distilling the hydrocarbon compounds to produce liquid fuel products, such as a naphtha (raw gasoline), a kerosene, a gas oil, and a wax, has recently been developed.
Since the liquid fuel products using the FT synthesis hydrocarbons as a feedstock have a high paraffin content, and hardly include a sulfur component, for example, as shown in Patent Document 1, the liquid fuel products attracts attention as environment-friendly fuels.
Meanwhile, in an FT synthesis reactor which performs the FT synthesis reaction, heavy FT synthesis hydrocarbons with a comparatively high carbon number is produced, and flow out as a liquid from a lower part of the FT synthesis reactor. In addition, light FT synthesis hydrocarbons with a comparatively low carbon number are generated involuntarily. The light FT synthesis hydrocarbons are discharged as gaseous by-products along with unreacted feedstock gas, from an upper part of the FT synthesis reactor.
Along with carbon dioxide, a steam, unreacted feedstock gas (carbon monoxide gas and hydrogen gas), and hydrocarbon compounds with a carbon number of 2 or less, hydrocarbon compounds with a carbon number of 3 or more which can be obtained as products (hereinafter referred to as “light FT hydrocarbons”) are included in the gaseous by-products.
Thus, conventionally, the gaseous by-products are cooled down to liquefy the light FT hydrocarbons, and then the light FT hydrocarbons are separated from the other gas components by a gas-liquid separator.